Proposals have already been made to heat diesel oil, particularly for cold starting diesel engines, for reducing emissions (white smoke on starting) by eliminating crystallization phenomena of paraffin products (needle crystals and nitrogen oxides) and, possibly, for saving fuel. The proper temperature is generally about 40.degree. C. (105.degree. F.). The various different methods are devices described on the subject indicate that means for heating the diesel oil are turned off once the oil admitted to the injection pump has reached a threshold temperature of about said desired value. Most heating systems either use an electrical resistance which is disconnected once the desired temperature is reached, or else they use radiator water whose temperature should never exceed about 80.degree. C. Some works even indicate that exceessive heating leads to certain drawbacks which should be avoided (see U.S. Pat. No. 3,913,543). Although there are works which suggest that preheating diesel oil enables vehicle fuel consumption to be reduced on the road, tests have shown that this is to be detriment of other performance features, and that fuel consumption per horse power hour remains substantially unchanged.